1. Field of the Invention
The invention relates to a method for calibrating a rotary encoder, particularly an incremental encoder, of an electric machine, the encoder outputting a sine track and a cosine track. The invention relates further to an encoder evaluation for carrying out the method and to an electric machine.
2. Description of the Background Art
In machines driven by an electric motor such as, for example, machine tools, the current phase position of the electric motor must be known for the precise operation of the machine. Thus, for example, the accurate metering of plastic in an injection molding machine or the correct pressure by an offset printing press supplied with paper rolls is only possible, if the current angle of the rotor with respect to the stator and/or the current rotational speed of the electric motor are known. Therefore, a rotary encoder which is configured as an incremental encoder and has a specific line count, for example, 256, is typically flange-mounted on a rotor shaft. Consequently, during a rotation of the rotor by 360°, 256 pulses are determined by the incremental encoder. In order to achieve an angular resolution of less than 1.4° (360°/256) as well, a sine and cosine track whose respective period corresponds to a rotation of the rotor by 1.4° is output by the rotary encoder. The sine or cosine track is typically recorded by two photoelectric or magnetic sensors, offset to one another by 90° with respect to their period, therefore in this case 0.35° of the rotor axis.
Because of fabrication tolerances of the rotary encoder, however, the tracks provided by the two sensors do not correspond to an optimal sine or cosine function. It is therefore necessary to calibrate the rotary encoder before the electric machine is operated. Amplitude, phase, and offset errors are differentiated in this regard. In a plot of rotary encoder-generated measuring points of the two tracks in a graph, whereby the x-coordinate of each plotted point corresponds to the measured value of the cosine track and the y-coordinate to the measured value of the sine track, the measuring points of an optimal rotary encoder would lie on a unit circle around the origin. When the unit circle is deformed to form an ellipse, there is accordingly an amplitude error, when the ellipse is tilted there is a phase error, and when the circle is shifted with respect to the origin an offset error of the rotary encoder.
DE 101 63 504 B4 discloses a method for calibrating the rotary encoder. In this case, a measured value is generated from the two tracks by taking the root of the sum of the squares of the two tracks; therefore the radius of the circle is determined provided the measured points were plotted in the aforementioned graph. In a further step, the time course of the determined radius is analyzed iteratively by means of a Fourier transform and correction factors are determined from this to eliminate the particular error. The circle resulting with the corrected tracks in the graph consequently has a center at the origin and a constant radius. In other words, the amplitude of the corrected tracks is the same, the offset of each track is equal to 0, and the phase between the two tracks corresponds to 90°. In other words, the cosine track corresponds to the sine track shifted by 90°.